This invention relates to screw rotors with a specific tooth profile suitable for use with screw compressors, and more particularly it is concerned with the shape and configuration of a screw rotor capable of performing hobbing.
Generally, a screw compressor comprises a male rotor member and a female rotor member forming a pair and maintained in meshing engagement with each other rotatably supported in a casing formed with an inlet port and an outlet port. This type of screw compressor generally uses a screw rotor of a tooth profile of nonsymmetrical type in which the forward face of the rotor and the backward face thereof differ from each other in shape and configuration.
Since this screw rotor of the nonsymmetrical type has a complex rotor profile, various problems have been raised with regard to improvement in the performance of the compressor and its production technology. With regard to the improvement of its operation performance, it is necessary that in addition to increasing the dimensional accuracy of the rotor members and casing, the length of the seal line constituted by the tooth profile and the area of the blow holes be taken into consideration.
In order to achieve these improvements, proposals have been made to use novel rotor profiles as shown in U.S. Pat. Nos. 4,140,445 and 3,787,154, for example. The rotor profiles shown in the prior art are primarily intended to provide improvements in operation performance by minimizing the blow holes, for example, in solving the problems with regard to operation performance. It is believed, however, that the problems with regard to operation performance and production technology have not thoroughly been studied and satisfactory solutions therefor have not been proposed. Let us set forth our views in greater detail in this respect. First, concerning operation performance, a problem would be raised with regard to the length of a seal line that would influence the operation performance of a screw compressor. The length of the seal line that is produced between the rotor members has particular bearing on the leak area between the rotor members, and when the seal line has a relatively large length, the leakage increases, thereby causing a reduction in the performance characteristics of the compressor. When the blow holes are large in area, the fluid would leak from the high pressure chamber side to the low pressure chamber side, thereby causing a reduction in the performance characteristics of the compressor. Additionally, the tooth profile is preferably such that the influences exerted by the degree of precision with which the tooth profile of the rotor members is finished on the operation performance of the compressor are minimized. Stated differently, the tooth profile is preferably such that the operation performance is not readily influenced by the degree of precision of the finishes given to the rotor members.
Concerning the production technology, it is desired that an improved process be developed which, as compared with a production process relying on a single cutter of the prior art, is capable of producing a screw rotor and which is superior to the prior art process in productivity and precision of finishes given to the screw rotor so that it is suitable for performing hobbing. Such process is further preferably capable of producing a screw rotor with a high degree of precision finishes at low cost, with the tools having high dimensional accuracy and a prolonged service life.
The problems stated hereinabove have been pointed out in the U.S. patents referred to hereinabove and proposals have been made to provide improvements for the purpose of obviating the problems. However, as it stands now, no satisfactory proposals have ever been made to provide a tooth profile which is capable of simultaneously meeting the requirements of solving the problems of how to improve operation performance and of improving production technology.
In this type of screw rotor of the nonsymmetrical type, when the rotor members mesh with each other or when the force of rotation is transmitted at a pressure angle .alpha. with the forward face flank of the male rotor member and the forward face flank of the female rotor member meshing with each other at a certain point, the force of rotation acts as a normal component of force of the tooth surface and a radial component of force of the rotor. It would be impossible to disregard the fact that these components of force manifest themselves as mechanical losses occurring between the tooth surfaces of the rotor members or in the bearings of the rotor.